Electrochemical (EC) capacitors have the properties of high power density, high reliability, high efficiency and a long lifetime. The applications areas of EC capacitors include trimming the blades of windmills as winds change and energy storage generated by solar panels, which are in the area of renewable energy. EC capacitors can be used in the energy storage in the area of transportation. When the electric buses run without power lines, the EC capacitors are quickly recharged whenever a bus is at any bus stop and fully charged in the terminus. This is the same in hybrid electric vehicles (HEVs) for using EC capacitors as the energy storage devices during regenerative braking. EC capacitors are being used in digital cameras to give a burst of power for flashes in the area of consumer electronics. EC capacitors can also be used for the short bursts of power needed in equipment designed for heavy lifting because they provide a longer operating life than the traditional lead acid batteries.
Numerous new applications of EC capacitors have been found in automotive and utility as energy storage components. Utilities have interest in EC capacitors as replacements for battery banks that are being used to buffer short-term outrages on the power grid. Probably the most pervasive application of EC capacitors as power components is starting to appear in the automobiles which are powered by the fuel cell. Some of these automobile are being manufactured by Honda Motor Company and also by Toyota, General Motors and others for lease to cities in the United States and elsewhere. EC capacitors are good at providing precise bursts of energy and also at receiving and storing energy bursts produced by regenerative braking.
However, regardless of the advantages and many applications of EC capacitors, the biggest challenge presented by EC capacitors is how to significantly increase the energy density which is now less than 10% of that in advanced rechargeable batteries. For a long period of time, a large amount of work has been done in understanding the relationship of the pore size to the ionic accessibility from the electrolyte and developing various pseudo-capacitance materials in order to maximize the charge storage capability1-4; however, there are limited studies on charge storage mechanisms such as the active role the electrolyte plays during the charge and discharge process: ion separation or ion shutter. In double-layer capacitors, the ionic concentration in the electrolyte increases and decreased during charge and discharge, respectively.
The energy density theory guide clearly shows that the energy densities for both double-layer capacitors and asymmetrical cells are mainly limited, by how many ions are available in the electrolyte or the salt concentration in the electrolyte5-9, because the minimum amount of required ions in the electrolyte is equal to the maximum charge capacity of the electrode in a capacitor. In contrast, for lithium (Li)-ion batteries, the Li ions shutter between two electrodes and the concentration keeps a constant value during charge and discharge; therefore, a high energy density cell can be obtained.
From the theoretical and experimental studies, it has been concluded that the energy density for both conventional EC double-layer capacitors or asymmetrical cells must be much less than that of advanced batteries, due to the fundamental difference between these two systems, in which the EC double-layer capacitors and asymmetrical cells consume the salt in electrolyte during the charge process; but the advanced batteries do not consume salt in the electrolyte. In EC double-layer capacitors and asymmetrical cells, the minimum weight of the required electrolyte in the cell is even greater than the weight of both electrode materials; however; in advanced batteries, the ion concentration in the electrolyte remains constant during the entire charge and discharge process, and there is no net ion exchange between the electrode and the electrolyte.
In recent years considerable reseal10-30 has been focused on the development of high energy density EC capacitors. Among all the energy storage systems that have been investigated and developed in the last few years, Lithium-ion Capacitors (LICs) have emerged to be one of the most promising, because LICs achieve higher energy density than conventional Electric Double-Layer Capacitors (EDLCs), and better power performance than Li-ion batteries (LIBs) as well being capable of long cycle life. LICs contain a pre-lithiated LIB anode electrode and an EDLC cathode electrode7-9. Previously, we have reported a LIC with activated carbon (AC) cathode and hard carbon (HC)/stabilized lithium metal powder (SLMP) anode electrodes with high energy density, high power density and long cycle life31-36.
However, to the best of our knowledge, there are not adequate reports about the low temperature performance of the LICs that can be discharged at −40° C.; while the low and high temperature performance of LIBs has been investigated tremendously by some research group in the past few years. Zhang and Kang et al.37-39 found the salt LiBOB and LiDFOB could be used as the additive to improve the low and high temperature performance of the LIBs; Smart et al.40-42 has also developed and compared numerous wide operating temperature range electrolyte formulations for the LIBs.
Therefore, in order to develop higher performance LICs with wider working temperature, more research work is needed to develop wide temperature range electrolyte that can be used in LICs. In this patent application, the inventors wish to report two types of wide operating temperature electrolyte formulas that contain methyl butyrate (MB) and additives, and enable the LICs to discharge at the temperature as low as −40° C. The electrochemical performance of the LICs with all kinds of electrolyte at various temperatures from 70° C. to −40° C. is studied and compared in detail. The cycling performance of the wide operating temperature electrolyte is also included in this patent application.
In this respect, before explaining at least one embodiment of the invention in detail it is to be understood that the invention is not limited in its application to the details of chemistry, composition, and to the functionality of the formulations set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. In addition, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be considered to be, or regarded as limiting.